Document deals with the carbon cost of global logging, which is, after agriculture, the human activity that has most reduced carbon stocks in vegetation and soil. Although felled wood releases carbon into the atmosphere at different stages, growing trees absorb this carbon, which has led to various carbon accounting methods for wood use. These methods often give the impression of low, zero or even negative greenhouse gas emissions from wood harvesting because in different ways compensate for carbon losses from new mining by sequestering carbon from the growth of large forest areas. However, the authors argue that attributing this sequestration to new logging is inappropriate because this forest growth would have occurred regardless of the new logging, for example due to agricultural land abandonment, restoration after previous logging, and climate change itself. On the other hand, some studies calculate gross annual emissions, thus not taking into account the ability of newly logged forests to regrow and approach the carbon stocks of unmanaged forests.

The authors present the results of the new model (CHARM – carbon harvest model), which uses time discounting to estimate the current and future carbon costs of global timber harvesting under different scenarios. They found that forest harvesting between 2010 and 2050 is likely to have annual carbon costs 3.5 – 4.2 Gt CO2e yr−1, which is close to common estimates of annual emissions from land-use change due to agricultural land expansion. Their study suggests an underappreciated opportunity to address climate change by reducing these costs.

The document further discusses the different contexts in which the greenhouse gas effects of forest harvesting are considered: life cycle calculations of wood products, national reporting of greenhouse gas emissions by governments, and scientific analyses assessing emissions from land-use changeMost common approaches have the common characteristic that carbon gains from tree regrowth from previous land management – and sometimes the further growth of unmanaged trees – compensate for carbon losses caused by new miningFor example, life cycle analyses of wood products or wood-based bioenergy commonly consider wood use as "carbon neutral" provided that the harvested forests are managed “sustainably”. Carbon neutrality in this context means that the carbon present in vegetation (biogenic carbon) and emitted at various stages as a result of harvesting, such as from decaying roots and litter, and from burning for fuel, as waste or at the end of life, is not counted. Although a definition of sustainability is often lacking, the common view is that forest harvesting is sustainable and carbon neutral if it does not exceed the annual increment of the “forest” (sometimes defined as an entire country). Some studies even consider the storage of even a small part of this carbon in durable wood products as contributing to carbon sequestration. Some studies even attribute the average carbon stock in the forests from which the wood comes to the harvesting and use of wood, leading to the conclusion that not only is logging not carbon neutral, but it contributes to carbon storage and benefits the climate.

On national level countries report the impacts of forestry using net accesses, which may create a similar impression of carbon neutrality. Due to the difficulties in separating the impacts of human management from natural changes in forests, the guidelines Intergovernmental Panel on Climate Change (IPCC) allow countries to report all changes in carbon stocks in “managed” forests as emissions or removals of carbon from the atmosphere. Managed forests account for three-quarters of the world’s forests. This approach allows countries to “account” for forest regeneration following abandonment of agricultural land or previous logging (even logging before international climate agreements set 1990 as the reference year). This approach also allows countries to take credit for accelerated growth in their forests as a result of CO2 fertilization effect, warmer weather and nitrogen depositionAlthough logging should reduce the nationally reported carbon balance under this approach, because the effects of logging are not reported separately, these reports may give the impression that logging in countries with a net increase in forest carbon stocks has no emissions.

Unlike national news, scientific studies estimating emissions from land use change try to separate the impacts of climate change on the carbon balance of forests as a “residual terrestrial carbon balance”, but they can still give a similar impression. Many studies only show net effects of new logging and restoration after previous logging, and therefore do not identify separate effects of new timber harvesting.

These forms of accounting have strong regional implications. Most forests in temperate zone is recovering from extensive logging or deforestation in the past, which reduced the need to feed horses and other draft animals and shifted agricultural land to the tropics. Conversely, tropical landscapes Overall, they see an expansion of agricultural land and an increase in forest harvesting. The net approach may therefore give the impression that logging in temperate, developed countries has zero or even beneficial climate impacts, while logging in developing, tropical countries is costly.

The authors emphasize that these forms of accounting do not accurately capture the impacts of new forest harvesting, because the forest growth and restoration used to offset the impacts of new logging would have occurred anyway. As hundreds of scientists and many scientific bodies have pointed out, any forest growth or restoration that would have occurred anyway cannot logically change the climate consequences of new logging.

On the other hand, some studies show gross emissions from logging, often in studies of the tropics. Although gross emissions are important, they are not considered an adequate measure of the climate costs of logging because they do not take into account potential recovery after logging. After logging, forests are likely to start to compensate for the lost carbon by growing faster than the same forests if they had not been logged, although they rarely catch up completely.

These losses in the short or medium term – in addition to long-term losses – undermine the goals of the Paris Agreement and are at odds with the legitimate commitments of many governments to achieve carbon neutrality by 2050 to avoid dangerous climate change. Given the importance of reducing emissions in the near term, European and American governments are requiring that the climate effects of direct or indirect land-use change due to bioenergy be assessed over a 20 or 30 year horizon. According to the authors, it makes sense to also give more weight to short-term emissions and mitigation when assessing the impacts of deforestation.

In the section about taking time into account when estimating greenhouse gas costs the authors describe the use time discounting to estimate the value of carbon losses due to past and likely future forest harvesting from 2010 to 2050 under different supply and demand scenarios. They use a new global forest carbon model, CHARM, which is based on the long-established approach of calculating the impact of timber harvesting on changes in atmospheric carbon over time as a transfer of carbon between different traysThese reservoirs include living vegetation, roots, litter, various wood products, and landfills. The impact on atmospheric carbon is the difference between the carbon stored in all reservoirs as a result of logging and the carbon that forests would store if they were not logged and continued to grow.

On valuation of the cost of one ton of emissions, equivalent to the value of a ton of mitigation, the authors use discount rate 4% to emissions and removals over time resulting from annual timber harvesting. This approach effectively transforms the value of the stream of emissions and removals in future years resulting from timber harvesting into "emissions equivalent to a year of mining".

The authors critically evaluate previous time-based approaches to life cycle analyses of wood products and biofuels. They emphasize the importance social cost of carbon (SCC), which estimate the changing real economic cost of emissions over time. Their discount rate 4% is consistent with the estimate of constant SCCs and 4% real rate of return on capital. The discounted carbon cost can be considered as "rental costs" of carbon.

The document also addresses growing demand for wood and presents a model for predicting future wood consumption according to four broad categories of wood products: long-lived products (LLP), short-lived products (SLP), very short-lived products – wood fuel (VSLP–WFL), and very short-lived products – industrial (VSLP–IND). The model assumes 54% increase in global timber harvest between 2010 and 2050.

The authors estimate annual carbon cost of global timber harvesting from 2010 to 2050 at 3.5–4.2 Gt CO2e yr−1 using a 4% discount rate for 40 years after each timber harvest. Existing levels of timber demand represent 78% of carbon cost, with the remainder attributable to growing demand. Industrial timber and wood fuel account for approximately half of the carbon cost.

The study also estimates "substitution value" based on the reduction of “production emissions” when using wood instead of concrete and steel in construction or when replacing traditional bioenergy. Global savings from this substitution are estimated at 0.8 to 0.9 Gt CO2e yr−1. However, the authors caution that this estimate does not take into account changes in forest carbon stocks and therefore may not mean that using wood causes fewer emissions overall than concrete and steel.

It is estimated that in different scenarios, the equivalent of logging would be harvested 756–855 million hectares soil.

Despite many uncertainties sensitivity analyses support the basic finding that forest harvesting causes approximately 3–5 Gt CO2e yr−1 when focusing on decadal effects. The results are likely conservative because they do not include the impacts of logging on soil carbon and the indirect impacts of forestry.

Using small or large discount rates or extending the forest payback period to 100 years has surprisingly modest effects on carbon costs.

The authors emphasize that their analysis is to some extent agnostic to economic influences on forest stands and their management, as their scenarios provide boundaries for possible future responses to economic forces.

In conclusion, the authors state that their estimates indicate that ongoing and likely increased logging has significant, though often ignored, carbon costs, which should be attributed to human activity. These estimated costs are similar to conventional estimates of annual emissions from land-use change due to agricultural land expansion. They see this finding as potentially good news because it suggests that if deforestation could be reduced, forest growth could contribute more to reducing atmospheric carbon, which represents a potential mitigation “wedge” that is rarely identified in climate strategies.

The methodological part of the document describes in detail model CHARM, its structure, input data (including data on consumption, harvesting and trade of wood from FAOSTAT, data on forest growth, the ratio of root to aboveground biomass, the rate of carbon decomposition in different pools), calculation of carbon costs and the time discounting method. The model distinguishes between forests from existing plantations and secondary forests. When estimating future demand for wood, the model uses fixed-effects model based on historical relationships between wood product consumption and population, GDP, and time.

The document also describes the calculation substitution values for replacing concrete and steel with wood and replacing fossil fuels with wood fuel. Spring

The paper was published in nature.com

Glossary of key terms

• Biogenic carbon: Carbon stored in living and recently dead organic matter. Biogenic carbon emissions come from biomass burning, organic matter decomposition, etc.
• Carbon neutrality (Carbon neutral): A state where net carbon emissions to the atmosphere are zero, meaning that any carbon release is offset by its removal (e.g., sequestration).
• Carbon sequestration: The process of removing carbon dioxide from the atmosphere and storing it for the long term in natural or artificial reservoirs (e.g., forests, soil, geological formations).
• Netting approaches: Methods of accounting for greenhouse gas emissions that take into account both sources of emissions and sinks and report a net result. In the context of forestry, this often means subtracting carbon gains in forests from emissions from logging.
• Time discounting: An economic concept that assigns a higher value to benefits and costs that occur sooner than to those that occur later. In the context of climate change, it is used to weight emissions and mitigation over different time horizons.
• CHARM (Carbon Harvest Model): A biophysical model developed by the study authors to estimate the impacts of logging on greenhouse gas emissions and land use.
• Long-lived products (LLP): A category of long-lasting wood products, such as lumber, wood panels, and other industrial logs used in construction and furniture making.
• Short-lived products (SLP – Short-lived products): A category of short-lived wood products such as paper and cardboard.
• Very-short-lived products – wood fuel (VSLP–WFL): Wood that is harvested directly for energy purposes.
• Very-short-lived products–industrial (VSLP–IND): Wood waste from the production of other wood products that is burned for energy.
• Annualized carbon costs: Total carbon costs spread evenly over the analysis period (in this study 2010-2050).
• Clear-cut equivalents: The rate of estimated harvest area, expressed as the area that would have to be clear-cut to obtain the same amount of timber, taking into account existing harvesting efficiency.
• Substitution value: Estimated reduction in emissions from the production of other products (e.g., concrete, steel, propane) when wood is used instead.